There has conventionally been known a working vehicle capable of traveling by a hydrostatic driving mechanism (hereinafter referred to as “HST”), the working vehicle being a vehicle mounted with a continuous stepless transmission including a variable capacity hydraulic pump driven by an engine and a variable capacity hydraulic motor rotated by means of pressured oil received from the variable capacity hydraulic pump, in which the speed of the working vehicle can be steplessly changed by changing the angle of a swash plate of either the variable capacity hydraulic pump or the variable capacity hydraulic motor.
Further, there has been an example of a speed changing device for a bulldozer as a working vehicle, which is, however, not a vehicle mounted with an HST, in which an operation grip of a mono-lever is provided with a speed-stage-switching switch for shifting up or shifting down the speed stage of a transmission. Either a shift-up operation signal or a shift-down operation signal output by the speed-stage-switching switch is input to a controller, and the speed can be changed to a second speed or a third speed by performing the shift-up operation with the speed-stage-switching switch (refer to, for example, Patent Document 1).
The speed changing device of mono-lever type may be applied to a bulldozer mounted with an HST. In other words, the configuration may be the one in which the bulldozer is enabled to travel by an HST, a controller for speed change is provided, and the tilting angle of the swash plate of either the variable capacity hydraulic pump or the variable capacity hydraulic motor is allowed to be continuously changed by operating the speed-stage-switching switch.
With the above configuration, when the speed-stage-switching switch provided on the operation grip of the mono-lever is pushed once, the angle of the swash plate is changed by one stage, which corresponds to a predetermined small angle, and the angle of the swash plate can be substantially continuously changed stage by stage by continuously pushing the speed-stage-switching switch once by once. The operator pushes the speed-stage-switching switch for a few times until the speed change ratio suitable to a desired vehicle speed is reached. Also, the speed change ratio can change fractionally and substantially continuously stage by stage if the speed-stage-switching switch is continuously pushed until the desired vehicle speed is reached, so that the substantially stepless speed change utilizing the features of HST can be achieved.
[Patent Document 1] Japanese Patent Publication No. 3,352,041 (Pages 4 to 5 and FIG. 3)